1. Field of the Invention
The present invention relates to a method for decontamination of contaminated soils and associated groundwaters, in general, and, in particular, to a method for treating soils contaminated toxic with radicals or compounds containing one or more higher valence state metals.
2. Description of the Prior Art
In the past, there have been proposed a variety of methods for decontaminating contaminated soil sites including, inter alia, industrial chemical by-product dumping sites and mining sites. The contaminating substances disposed at such sites include petrochemicals, coal tar chemicals, penta chloro phenol, arsenic compounds, sulfonated organic compounds and a host of others. The methods that have been suggested and/or practiced can be classified under a number of categories including: treatment without moving the soil on site nor removing the soil from the site, thermal treatment with soil removal from the site, and biological treatment with on-site soil movement.
In the category of treatment without moving the soil on-site nor removing the soil from the site there has been proposed the following methods.
First, it has been proposed to electrically heat a block of soil until melted to a glassy state. While such a method may in fact decontaminate the soil, it undesirably and permanently changes the physical characteristics of the soil.
Second, it is known to enclose a decontaminated area with a cement slurry wall and a clay cap. This method does nothing to reduce or eliminate the contamination but merely serves to contain the contamination. Such a method is prone to high ongoing maintenance costs due to periodic and required testing of the region surrounding the contaminated area to determine if contaminants had escaped from the area and the costs of restoring the slurry wall and/or clay cap if leakage had occurred.
Third, it is known to place interceptor water wells around the perimeter of a contaminated area, then pump the contaminated water out from the contaminated area until the water within the area attains a low elevation, and then biologically treat the contaminated water pumped from the wells. Within this method, there are a series of further treatment options which are available.
A first option is to discard the biologically treated water. With this option, groundwater inflow from outside the perimeter area will keep the contamination "bottled-up" within the boundaries of the contaminated area and will, with time, dilute the concentration of contaminants within the contaminated area to an acceptable level. However, no time prediction can be projected for when the site will be virtually free from contamination.
A second option is to re-inject the biologically treated water into the contaminated site in order to move the soil contamination toward the perimeter wells. With this option, clean-up could be erratic and unpredictable due to possible pockets of extremely impervious soil which would lead to uneven groundwater flows.
A third option is to inject biological sludge and oxygen into wells in the contaminated area in the hope of inducing biological activity underground. With this option, unfortunately, the induction of biological activity is not always successful.
A fourth option is to inject solvents, oxygen, steam and other reactive chemicals into the contaminated area and possibly generate reduced pressure in order to decontaminate by chemical reaction. A disadvantage of this option is that it is very inexact and unpredictable.
In the category of thermal treatment with soil removal from the site there has been proposed the use of rotary kilns and externally fired heat treaters.
A rotary kiln has the drawback of mixing heating gases (combustion gases) with gases from the contaminated soil incineration process. This may require the scrubbing of a large volume of gases, and the composition of these gases may widely vary depending upon the contaminant loading in the soil being treated. In addition to the above, a rotary kiln can generate large dust loads in the exhaust gases due to the previously mentioned large volume of gas which occurs when the combustion gases mix with the incineration gases of the treated soil.
An externally fired heat treater is an alternative to the directly-fired rotary kiln. With an externally fired heat treater heat is supplied through the treatment vessel's walls and combustion gases do not come into contact with the soil being treated. However, some combustion gases of low oxygen content could be injected into the treater in order to remove high boiling organic, create a safe low O.sub.2 atmosphere, and carbonize any residual organic that do not volatize.
For the soil treatment methods that require the soil be removed, the soil would need to be transported to a treater, treated, and then finally transported to a site that may or may not be the final deposition location for the clean soil. The reason for the uncertainty of the final deposition location is that the water table can be high enough in some locations to cause a virtual mud field to exist so that keeping contamination away from clean soil would be difficult. Also, if the soil to be treated is removed from below the water table, then the water table needs to be lowered by pumping (with appropriate treatment and discard of the pumped water), otherwise the removed soil would be completely saturated with water. If no water was pumped, then the excess water could be drained by placing the treated soil well above the water table for a period of time. In either case, without removal of this excess water, the heat load on the thermal treater would be very high, causing a low through-put rate and high cost of operation of the treater.
If soil removal and treating is pursued, the overall problem reduces to removing contaminated soil, controlling the ground water level in the soil excavation area, treating any soil and water removed from the excavation area and then replacing the clean treated soil in such a manner that the treated soil does not become contaminated. In the performance of such a treatment operation, the method of actually treating the soil can become secondary to the method of soil handling. Furthermore, the cost of auxiliary work (for instance treating and disposing of a large volume of contaminated water) could exceed the cost of operating a soil treater such as a thermal unit.
In the category of biological treatment with on-site soil movement there has been proposed a method of flooding a contaminated dumpsite area to form a pond, using biological micro-organisms to treat the water and soil of the flooded dumpsite, mixing the contaminated soil of the dumpsite pond bottom into the micro- organism infested water, and backfilling the dumpsite upon completion of the biological treatment process. Such a method has proven to be an effective way of treating contaminated soil areas. However, the costs and logistics of supplying a quantity of fill material sufficient to backfill the site (which sometimes can be many acres in area and 20 or more feet in depth) can become quite prohibitive. It is upon this sort of method that the present invention represents a significant and cost-effective improvement.
In order to avoid the costs and logistical problems associated with the above described procedures, a procedure referred to as a "moving lake" biological treatment was suggested in U.S. Pat. No. 4,948,297. In this method, a body of water such as a lake is created on one edge of a contaminated area, if one does not already exist, and then, once biological activity has been established, the lake is caused to be slowly "moved" through the contaminated area. The lake would serve as a large biological treating system and any contaminant in the soil and water, if biologically consumable, would be consumed as the lake progressed through the contaminated area.
While the above described "moving lake" biological treatment can effectively treat contamination, it is known that metallic contamination frequently accompanies such organic contamination. U.S. Pat. No. 4,948,297 discloses that such metallic contamination may be controlled in the lake by passing filtered lake water over a bed a cation exchange resin. Such treatment can, however, be expensive. It is therefore, the object of the present invention to control such metallic contamination without the need to rely on or to fully rely on such cation exchange resins.